<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
	<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
	<title>Document</title>
	<style type="text/css">
		.bb{
			width: 200px;
			height: 200px;
			border: 1px solid blue;

		}
		.bb .bl{
			width: 50px;
			height: 50px;
			border:1px solid red;
		}
	</style>
</head>
<body>
<div class="bb">
	<div class="bl"></div>
	
</div>	
</body>
</html>